This research is concerned with obtaining a more precise understanding of relationships between neuroelectric measures of human brain function, as manifested by brain event-related potentials (ERPs) recorded from scalp, and behavioral measures of cognitive activity. ERPs are composed of a series of overlapping components, each relating to different aspects of information processing. More precise identification of the functional roles of ERP components provides a basis for 1) clarifying the timing and organization of various stages of cognitive mental operations; 2) providing a non-invasive method of obtaining information on the neurophysiological correlates of these mental operations; and 3) contributing to the understanding of a variety of conditions such as learning disabilities, mental retardation and neurological and psychiatric disorders. The experiments and data analysis procedures are designed to isolate and characterize components by a) establishing systematic relationships between differences in experimental conditions and differences in ERP components, and b) examining differences in scalp distribution. The experiments deal with a group of positive polarity components that reflect relatively late stages of evaluation of information provided by the eliciting events, and two very late long duration negative components that occur in conjunction with sustained conceptual mental operations. The specific aims are: 1) To contribute to the clarification of the necessary and sufficient conditions for eliciting P3b, a widely studied, robust component that relates to contextual evaluation of the eliciting stimulus; 2) To determine how the complexity dimension of information affects ERP components that are known to relate to other dimensions of information (relative probability of events and uncertainty arising from perceptual difficulty); 3) To sketch out the properties of what appears to be a new ERP component that is obtained in relation to the information that a subsequent task will be easy or difficult; 4) To examine late long duration components associated with sustained mental operations involving arithmetic computation and mental rotation of visual images.